Supernova Type Ib

Originally lumped in with Type Ia (SNIa) and Type Ic (SNIc) supernovae due to the similar appearance of their light curves, Type Ib supernovae (SNIb) were recognised as a separate class in the mid-1980s. This distinction was made based on the absence of the silicon absorption feature typical of Type Ia spectra, and the obvious presence of helium absorption which is absent in Type Ic spectra at maximum light. That SNIb should be classified separately to SNIa was further justified on examination of late-time spectra, where it was found that SNIb spectra were dominated by intermediate mass elements and SNIa spectra by heavy elements at these epochs.

Despite the similarities in light curve shape and early-time spectra, it turns out that SNIb have much more in common with Type II supernovae (SNII) and SNIc than SNIa. They are primarily found in arms of spiral galaxies close to HII regions, they emit radiation at radio wavelengths indicating the presence of circumstellar material surrounding the progenitor, and some SNII have been observed to transition to SNIb at late times! All of this leads to a model where SNIb also result from the core-collapse of a massive star, with the difference between SNII and SNIb being that the Type Ib progenitor lost its outer layer of hydrogen before the collapse. SNIc progenitors are one step further down the line, having lost both their hydrogen and helium outer layers before the core-collapse.